Photon detectors exploiting the photoelectric effect

Photon detectors are essential for many optical applications qualitative and quantitative light intensity measurements, image recognition and spectral measurements. The basic principal of any photodetector is that it converts light into electricity, and typically (though not always) the current response of the system is measured. 

The photoelectric effect is the emission of electrons from a material when it is exposed to a photon flux it was first discovered in the late 19th century, and it can be said that devices based on this effect were the first quantitative photon detectors (as early as 1902). 

Photoelectrons released from a surface by light impact have very low kinetic energy, and thus they only travel any appreciable distance in a high-vacuum environment. The photoelectric effect exhibits threshold behaviour, i.e. light below a certain frequency, which depends on the material of the cathode being exposed, will not result in the emission of electrons, regardless of how high the irradiance is. 

The simplest detector configuration consists of a photosensitive cathode, frequently shaped in the 

Photomultipliers are used in circumstances where very low light levels need to be detected ultimately, with careful suppression of system noise, single photons can be detected. 

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